In-Reply-To: <20080806152555.GD13996@kroah.com>
To: Greg KH <greg@kroah.com>
Cc: Eric Paris <eparis@redhat.com>, linux-kernel@vger.kernel.org,
       malware-list@lists.printk.net
Subject: Re: [malware-list] [RFC 0/5] [TALPA] Intro to a linux interface	for on
 access scanning
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From: tvrtko.ursulin@sophos.com
Date: Wed, 6 Aug 2008 17:03:20 +0100
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Greg KH <greg@kroah.com> wrote on 06/08/2008 16:25:55:

> On Wed, Aug 06, 2008 at 10:37:06AM +0100, tvrtko.ursulin@sophos.com 
wrote:
> > Greg KH wrote on 05/08/2008 21:15:35:
> > 
> > > > > > Perf win, why bothering looking for malware in /proc when it 
can't
> > > > > > exist?  It doesn't take longer it just takes time having to do
> > > > > > 
> > > > > > userspace -> kernel -> userspace -> kernel -> userspace
> > > > > > 
> > > > > > just to cat /proc/mounts, all of this could probably be 
alliviated 
> > if we
> > > > > > cached access on non block backed files but then we have to 
come 
> > up with
> > > > > > a way to exclude only nfs/cifs.  I'd rather list the FSs that 
> > don't need
> > > > > > scanning every time than those that do....
> > > > > 
> > > > > How long does this whole process take?  Seriously is it worth 
the 
> > added
> > > > > kernel code for something that is not measurable?
> > > > 
> > > > Is it worth having 2 context switches for every open when none are
> > > > needed?  I plan to get numbers on that.
> > > 
> > > Compared to the real time it takes in the "virus engine"?  I bet 
it's
> > > totally lost in the noise.  Those things are huge beasts with 
thousands
> > > to hundreds of thousands of context switches.
> > 
> > No, because we are talking about a case here where we don't want to do 
any 
> > scanning. We want to detect if it is procfs (for example) as quickly 
as 
> > possible and don't do anything. Same goes for any other filesystem 
where 
> > it is not possible to store arbitrary user data.
> 
> See previous messages about namespaces and paths for trying to figure 
this
> kind of information out in a sane way within the kernel.

How are namespaces and pathnames relevant to exclusions based on 
filesystem *type*? It is not about checking for /proc but checking if the 
filesystem name is proc, sysfs, ..
 
> > > > > > In kernel caching is clearly a huge perf win.
> > > > > 
> > > > > Why?  If the cache is also in userspace, it should be the same, 
> > right?
> > > > 
> > > > In kernel cache has 0 context switches for every open.  Userspace
> > > > caching has 2.  Every open has to block, switch to the context of 
the
> > > > userspace client/cache, get that decisions, and then switch back 
to 
> > the
> > > > original process.
> > > 
> > > Again, compared to what?  If you in userspace are doing big complex
> > > things, such an overhead is trivial.
> > 
> > Again similar thing as above - In case of a cache we are not doing 
complex 
> > things.
> 
> Except for the overhead of keeping a cache :)

As small as possible - just one extra field in the inode struct. 

And really you are talking about a different thing again and moving the 
argument around. I answered you why your argument about putting a cache in 
userspace since we do so much there already is wrong, because we don't do 
a lot (anything really) in userspace for vast majority of opens if we have 
this totally simple in-kernel caching scheme.
 
> > So I think you can't argue that because scanning is slow everything
> > else has to go to userspace. On a typical running system scanning is
> > exceptional and everything else benefits from being in the fast path.
> 
> I really can not judge as we have not seen an implementation yet.

Did you mean "_I_ have not seen an implementation yet"? Because Eric 
posted it so you can have a look at your leisure.

Tvrtko


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